This application claims the priority of German Patent 199 16 797.4, filed Apr. 14, 1999 and PCT International No. PCT/DE00/01105 filed Apr. 11, 2000, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to a semiconductor gas sensor and a process for measuring gas concentrations with a semiconductor gas sensor.
The measurement or analysis of gases is very important in different fields of technology. For example, during the combustion of fossil fuels, carbon monoxide, nitrogen oxides, and ozone are produced, which represent significant burdens on the environment and the human body. To reduce these loads, it is necessary to measure or analyze the gases generated during combustion. In particular, a measurement of the exhaust gases during the operation can reduce the emission of harmful substances by means of suitable feedback.
One possibility for gas analysis is offered by semiconductor gas sensors, where a gas sensitive layer, which changes its electric resistance when loaded with specific gases, is raised to a specific measurement temperature. By measuring the electric resistance of the sensitive layer at specific temperatures, different gas concentrations, for example of CO, NO, NO2, or O3, can be determined. In most cases the gas sensitive layer is a metal oxide layer, for example, of SnO2.
The article by B. Ruhland et al., xe2x80x9cGas Kinetic Interactions of Nitrous Oxides with SnO2 Surfacesxe2x80x9d, Sensors and Actuators, B50 (1998), pages 85 to 94, shows such a semiconductor gas sensor. In this prior art gas sensor, a thin layer of SnO2 is disposed on a heating structure. A SiO2 layer separates a heating element from the gas sensitive SnO2 layer. The heating structure with the gas sensitive layer is arranged on a Si3N4 membrane, which in turn is mounted on a silicon substrate. In the measurement process, the gas sensitive layer is loaded with the gas by means of diffusion or by flow in the direction of the sensor element. The sensor element with the gas sensitive layer is arranged in a housing.
However, this presents the problem that the gas diffusion in the direction of the sensor element is prevented by means of the raising warm air or the gas, rising above the sensor element. Thus, gas streams are generated in the housing that have a negative impact on the time response of the sensor. This results in long response times and to some extent inaccurate measurement results.
Therefore, the object of the present invention is to provide a semiconductor gas sensor, which exhibits an improved time response. Furthermore, the gas sensor exhibits a compact design and is able to be produced economically. In addition, a process for measuring gas concentrations is disclosed that shows an improved time response and enables accurate measurements.
The semiconductor gas sensor of the present invention includes a heatable sensor element for measuring gas concentrations and a housing, in whose interior is disposed the sensor element. The housing exhibits a first opening, which connects the interior to the exterior, and the housing has one or more second openings, which lie deeper than the first opening in order to drive a gas stream from the second opening to the first opening by means of convection. In this manner, the response times are reduced, while the measurement accuracy is high and the cost is low.
The second openings or the gas inlet openings are arranged on the side walls preferably in the bottom part of the housing; and the first opening can be arranged on the upper side of the housing. Preferably, the second openings are arranged on the same level or deeper than the sensor element. The result is optimal gas flow in the interior of the housing.
Preferably, the housing is made of silicon by means of micro engineering. In this respect, the sensor element can be integrated into two silicon troughs, which are stacked either one above the other or opposite each other. The second openings are formed between the mutual interfaces of the troughs. In particular, the first opening is formed in the top silicon trough over the sensor element. Preferably, the second openings, which form the gas inlet openings, are formed by means of passages or channels, located between the silicon troughs. This type of construction allows an especially simple and economical production, whereby the sensor can be realized so as to be extremely compact.
Perfusible elements for filtering or reaction of the gas can be disposed in the second opening(s) or gas inlet opening(s). In this respect, the inside surface of the perfusible element exhibit for example, a material that subjects the gas flowing through to a chemical and/or catalytic reaction, before said gas reaches the sensor element. Thus, the sensitivity of the sensor can be set or changed for different gases.
Another aspect of the invention provides a process for measuring gas concentrations with a semiconductor gas sensor, wherein a gas stream is driven by means of convection through a housing. The gas flows through the housing from the bottom to the top and is guided past a sensor element, which produces a measurement signal as a function of the gas concentration.
Other objects, advantages, and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.